INFILTRATION INTO SEALED SOILS AS INFLUENCED BY GRAVEL COVER

The influence of stone gravel on water infiltration is not thoroughly understood, in that previous studies led to contradictory results. This field work was conducted to determine the effect of gravel on infiltration into sealed soils and to develop a regression model for infiltration based on the areas of the exposed and covered surfaces. Data were collected on 11 soils from three great groups from the fringe of the Sahara to the wet savanna zone of West Africa. A portable rainfall simulator was used to simulate rainfall on 28 undisturbed 1 by 1 m plots. Surface slope (P), portions of surface areas covered by grass vegetation (V), coarse fragments partly embedded in a surface seal (E), free coarse fragments (F), and sealed patches (S) were assessed as well as mean diameters (MD) of coarse fragments. The surface coverage by free surface gravel alone accounted for 68% of the variance of the cumulative infiltration/rainfall ratio, denoted as the infiltration ratio (10). A simple nonlinear model taking into account S, F, and MD resulted in a high coefficient of determination of 10 (R2 = 0.95). This model tested with three other sets of data was regarded as satisfactory to predict 10 (r2 = 0.87). The results suggest that infiltration rate (IR) could be better related to the effective infiltrating annulus (AN) along the perimeter of free gravel than to the whole area under the free gravel (F). Moreover, simulated results indicated that IR increased with increasing F for MD < 0.029 m. For higher values of MD, IR decreased with increasing F. Thus, in the regions where large gravel-size coarse fragments prevail, infiltration decreases with increasing stone cover, especially if the stones are embedded in a surface seal, as in desert pavements. Conversely, infiltration is enhanced by stone cover where small free coarse fragments are dominant, like those originating from a dismantled Fe pan.